1. Field of the Invention
This invention is concerned with temporary shutdown of CO-combustion devices fed with flue gas produced in the catalytic cracking of petroleum hydrocarbons. In particular, it is concerned with temporary shutdown of CO-boilers and CO-incinerators in petroleum refineries.
2. Description of the Prior Art
Catalytic cracking of petroleum fractions is a well-established refinery process. The catalytic cracking apparatus per se comprises a reactor section that contains a reaction zone where fresh feed is mixed with hot regenerated catalyst under cracking conditions to form cracked products and deactivated, coked catalyst; and a regenerator section that contains a regeneration zone where the coked catalyst, after separation from volatile hydrocarbons, is burned by contact with air to form regenerated catalyst. Moving catalyst bed and fluidized bed versions of this process are used. Regardless of the design of the catalytic cracking apparatus, all present-day plants operate with a catalyst inventory that continuously circulates between the reactor section and the regenerator section. The two sections are connected by conduits through which circulation is maintained.
It is common practice to operate the regenerator with a limited amount of air feed so that the gaseous combustion products contain less than about 0.2 volume percent oxygen. Under such conditions, substantial concentrations of carbon monoxide (CO) are contained in the flue gas exiting from the regenerator. The actual concentration of carbon monoxide in the flue gas may vary depending on the particular plant, the nature of the catalyst and the detailed operation of the regenerator, but usually it remains in the range of about 4 to about 9 volume percent. The volume ratio of carbon dioxide to carbon monoxide (i.e. CO.sub.2 /CO ratio) normally varies from about 0.7 to about 3, and is a measure of the completeness of combustion of the reacted carbon in the coke. Thus, in operating with a limited amount of air, only about three-fourths of the total potential heat of combustion of coke is released in the regenerator itself.
Many refineries continuously feed the flue gas to a CO-boiler to complete the conversion of CO to CO.sub.2, and thus generate substantial quantities of process steam for use in the cracking process or elsewhere in the refinery. In general, the CO-boilers used differ in design from refinery to refinery, but they are generally utility boilers of the tube type. In operation, the flue gas is enriched with air and burned in the furnace of the boiler. The boiler ordinarily is equipped to accept at least one other fuel, which is used in start-up, or to supplement the fuel value of the flue gas, or to provide process steam when the catalytic cracking apparatus itself is shut down. Because of the nature of the service, the operation of the CO-boiler is subject to temporary shutdown for maintenance and repair. During these periods of shutdown, there is usually no other available means to reduce the CO content of the flue gas from the regenerator of the catalytic cracking process. In many communities this creates a serious problem because of antipollution regulations. Depending on circumstances, the catalytic cracking apparatus itself may have to be shut down, or permission of the civil authorities may be required to operate temporarily out of compliance with the ordinances.
In some refineries, the flue gas is passed to a carbon monoxide incinerator (CO-incinerator) where the CO is burned to CO.sub.2. Here again, temporary shutdown of the incinerator for maintenance or repair creates a problem in the disposal of the flue gas, which may in some cases be resolved only by also shutting down the catalytic cracking operation itself. Such shutdown is complex and costly.
For convenience, the term "CO-combustion device" will be used in this specification, including claims, to refer to either a CO-boiler or a CO-incinerator, since both of these units serve to combust CO to CO.sub.2.
It has been known for some time that cracking catalysts may be modified by the addition of metal combustion promoters to increase the CO.sub.2 /CO ratio, and thus the combustion efficiency in the regenerator. The use of chromium as a promoter for moving-bed type catalytic cracking catalysts is one such example, more fully described in U.S. Pat. No. 2,647,860. In fact, a number of other metals, including nickel, deposited from the feedstock to the cracking process, are also believed to effect some degree of change in the combustion efficiency. Up until recently, however, most of the known metals had the serious drawback that, when included in the cracking catalyst in sufficient quantity to substantially effect the combustion efficiency, they also had a substantial detrimental effect on the cracking selectivity. It is well recognized, for example, that more than extremely small trace amounts of nickel in the feedstock to the cracking unit cause excessive production of coke and dry gas.
It has recently been discovered that very substantial effect on the combustion efficiency can be achieved, with little or no effect, or even an advantage, in the cracking operation, if certain Group VIII metals, more fully described hereinafter, are added to the cracking catalyst. In fact, the operation of the regenerator can be changed from partial combustion of carbon to substantially complete combustion if the cracking catalyst is promoted with as little as 2 ppm of platinum, for example. This development is more fully described in co-pending U.S. application Ser. No. 649,261 filed Jan. 15, 1976, the entire contents of which are incorporated herein by reference.
As described therein, the metal combustion promoter may be deposited on the cracking catalyst during operation of the catalytic cracking process by introducing a suitable compound of the metal into the hydrocarbon feed to the cracking zone. This method of depositing the combustion promoter on the cracking catalyst has the particular advantage that it can be done rapidly, and thus rapidly affect the combustion efficiency during burnoff of coke deposits in the regenerator.